Giant magnetoresistive (GMR) and tunneling magnetoresistive (TMR) devices have been developed for high density data storage applications. Both GMR and TMR devices feature a reader stack of multiple layers that include a nonmagnetic spacer layer between two ferromagnetic layers. Typically, one of the ferromagnetic layers acts as a reference or pinned layer having a fixed magnetization, while the other ferromagnetic layer referred to as a free layer has a magnetization that rotates in response to an external magnetic field. In a GMR device, the nonmagnetic spacer layer is electrically conductive. In a TMR device, the spacer layer is a very thin electrically insulating layer that forms a tunnel barrier between the free layer and the reference layer.
TMR reader stacks using magnesium oxide (MgO) have been used in the commercial hard drives with area density up to 500 Gb/in2. As the area density further increases, the reader size (both reader width and reader stripe height) must decrease. This forces a reduction in the product of resistance and area (the RA product) for the MgO TMR stacks in order to maintain the same reader resistance. However, the reduction of RA for the MgO stack not only significantly decreases the TMR value, but also greatly increases the coupling field between the free layer and the reference layer in the TMR stack (the free layer H1 value). For example, the H1 value will go up to about 300 Oe when the RA is about 0.6 Ωμm2, and up to about 500 Oe when the RA is about 0.4 Ωμm2. Such a high free layer H1 value is not acceptable in a magnetic head application, because it may shift the asymmetry mean and/or require an extremely thick permanent magnet (PM) in order to align the free layer parallel to the air bearing surface (ABS). A much thicker PM sacrifices the shield-to-shield spacing and reduces the area density.
To achieve high GMR reader stacks with moderate low RA (0.1˜0.4 Ωμm2), in past several years, research efforts have been directed to current-confined-path (CCP) current-perpendicular-to-plane (CPP) GMR readers for use in the hard drive industry. Examples of CCP-CPP GMR devices are described in Fukuzawa et al., US 2006/0050444; Fukuzawa et al., US 2006/0098353; Childress et al., US 2007/0047154; Carey et al., US 2007/0097558; Zhang et al., US 2007/0188936; Fuji et al., US 2008/0008909; Yuasa et al., US 2008/0026253; and Nowak et al. U.S. Pat. No. 7,093,347.
CCP-CPP readers may be made by doping some oxide particles into the spacer (like Cu) of the pure CPP stacks to increase the RA to moderate low RA (0.1˜0.4 Ωμm2) from very low RA (less than 0.1 Ωμm2) in the pure CPP stacks. The function of the doped oxide section is only to confine the current path to increase the RA. It has little contribution to increase the GMR or even deteriorate the GMR value. Furthermore, in the traditional CCP reader designs, it is very difficult to control the size of the conductive channels or oxide particles within the nanometer range, as well as to control the size variation. This problem may lead to very large sensor-to-sensor RA and GMR variation within a wafer and may result in significant yield reduction in the mass production of magnetic read/write heads. Making very small (in nanometer or even angstrom range) and uniform conductive channels or oxide particles inside of the spacer layer is a very difficult technical challenge.